1. Field of the Invention
The present invention relates to a developing device for developing a latent image formed on a latent image carrier, a process cartridge integrally including the developing device, and an image forming apparatus, such as a printer, a facsimile, a plotter, a multifunctional machine including one of the functions of these devices, including one of the process cartridge and the developing device.
2. Background of the Invention
In the past, a developing device employed in an image forming apparatus, such as a printer, a facsimile, a plotter, etc., employs one or two-component developing system. The two-component developing system is very suitable for high speed developing, and is mainly employed in a middle or high-speed image forming apparatus. To obtain a high quality image using the two component developing system, developer needs to be extra ordinarily accurate at a section contacting a latent image formed on the latent image carrier. Thus, a diameter of a carrier particle is decreasing these days, and carrier having a diameter of about 30 micrometer recently comes to be used on a commercial basis.
Since a mechanism can be downsized and lightened, the one component developer is mainly used currently in a low speed image forming apparatus. To form a thin toner layer on a developing roller in the one component developing system, a toner-adjusting member, such as a blade, a roller, etc., contacts toner on the developing roller, so that the toner is discharged by means of friction created between the developing roller and the toner adjustment member. Such a discharged toner thin layer on the developing roller is conveyed to a developing section and develops the latent image on a latent image carrier. A developing system employed here is categorized into two of contact and non-contact types, wherein the former represents that a developing roller contacts a latent image carrier, and the latter does not. As shown in the Japanese Patent Application Laid Open No. 03-100575, to complement defects of the two and one component developing systems, a hybrid system blending these two and one component developing systems has been proposed.
A method of developing a fine uniform dot with high resolution is disclosed in the Japanese Patent Application Laid Open No. 03-113474. The method employs the above-mentioned hybrid system and arranges a wire to which a high cycle bias is applied at a developing section, and causes toner clouding there, so that a dot developing performance is obtained with high resolution. The Japanese Patent Application Laid Open No. 03-21967 proposes a method of forming an electric field curtain on a roller to most efficiently and stably forming toner clouding. The Japanese Patent Application Laid Open No. 2003-15419 discloses a developing device for conveying developer by means of an electric field curtain formed by a traveling wave electric field. The Japanese Patent Application Laid Open No. 9-269661 discloses a developing device including plural magnetic poles, which almost equally attract almost one layer of carrier onto the periphery of a developing roller.
The Japanese Patent Application Laid Open No. 2003-84560 discloses a developing device that periodically (or intermittently) includes plural conductive electrode patterns on the surface of a developer carrier member that carries non-magnetic toner via an insulation section, and causes an inclination of an electric field in the vicinity of the surface of the developer carrier member when a prescribed bias voltage is applied to the electrode, so that the non-magnetic toner is conveyed being attracted to the developer carrier member.
A high quality image is increasingly demanded in the two component developing system. Thus, since a dot size of a pixel is necessarily equal or smaller than a diameter of a currently used carrier particle, the currently use carrier particle further needs to be downsized in view of a reproduction performance of an isolated dot. However, when the carrier diameter is decreased, a magnetic permeability of the carrier particle decreases. As a result, the carrier easily drops off the developing roller. When the dropped carrier particle sticks to the latent image carrier member, not only an image is defective due to toner sticking, but also side effects, such as damaging the latent image carrier member, etc., occur thereafter.
To prevent the carrier drop, many attempts have been made in a view point of material, such as increasing a magnetic permeability of carrier particle, intensifying a force of magnet installed in a developing roller, etc. However, since it is difficult to obtain a high quality image at low cost, development hardly is progressed. In view of a current demand for downsizing, a diameter of a developing roller is increasingly decreased steadily, designing of a developing roller having a strong magnetic field capable of completely suppressing carrier drop becomes difficult. From the beginning, since the two component developing system forms a toner image by rubbing ear of the two-component developer called a magnetic blush, development of an isolated dot tends to be uneven due to un-uniform ear. It is possible to improve an image quality by forming an alternating electric field between a developing roller and a latent image carrier member. However, image unevenness caused by uneven developer ear can hardly be removed completely.
In the one component developing system, since a toner layer thinned by a toner adjustment member on a developing roller has already been pressure contacted the developing roller, a response of the toner to an electric field is extremely slow at the developing section. Thus, forming an intense alternating current field between a developing roller and the latent image carrier is common to obtain a high quality image. However, it is still difficult to develop a latent image with a constant amount of toner, and to uniformly develop a fine dot with high resolution. Further, since the one component developing system causes extremely large stress in toner when the toner layer is thinned on the developing roller, the toner quickly deteriorates during circulation through the developing device. As the toner deteriorates, the toner layer is easily unevenly thinned on the developing roller. Accordingly, the one component developing system is generally not preferable for a high speed and durable image forming apparatus. Even a size and a number of parts of a developing device of a hybrid system increase, some problems can be resolved. However, the developing section also includes the same problem as that using the one component developing system. That is, it is difficult for it to develop a fine and uniform dot with high resolution.
The system of the Japanese patent Application Laid Open No. 03-113474 can realize highly stable and high quality development. However, the developing device becomes complicated. The system of the Japanese patent Application Laid Open No. 3-21967 is significantly excellent in obtaining downsized and high quality image. However, conditions for an electric field curtain and development are too strict to obtain an ideal high quality image. Specifically, when an image is formed on a wrong condition, an expected result does not occur, and a bad quality image is rather provided.
Further, in an image forming process that sequentially superimposes first to third toner images on a latent image carrier, a developing system is needed not to disturb a toner image already formed thereon. By employing either a non-contact type one component developing system or the toner cloud developing system as described in the Japanese Patent Application Laid Open No. 03-113474, respective color toner image can be subsequently formed on the latent image carrier. However, since an alternating current electric field is necessarily formed between the latent image carrier and the developing roller in the both systems, the toner image previously formed on the latent image carrier is partially peeled off and enters the developing device. As a result, an image on the latent image carrier is not only disturbed, but also colors of the toner in the developing device are blended. Such a problem is fatal in creating a high quality image. Thus, the cloud developing is to be realized while the alternating current electric field is not created between the latent image carrier and the developing layer to resolve such a problem.
As a cloud development-realizing method, a system described in the Japanese patent Application Laid Open No. 03-21967 can be effective. However, as mentioned earlier, the method needs to be used under the strict condition, or any of the expected result is not obtained. Further, as described in the Japanese Patent Application Laid Open No. 2002-341656, a method of electro-statically conveying toner using an alternating electric field having more than three phases while omitting mechanical driving for a toner carrier member can be effective. However, this method includes a problem of toner accumulation on a conveyance substrate triggered by toner impossible to electro-statically convey due to some reason. To resolve such a problem, a structure of a secured conveyance substrate and a toner carrier member that moves along the surface of the secured conveyance substrate has been proposed as discussed in the Japanese patent Application Laid open No. 2004-286837. However, a mechanism becomes extremely complex.
In the cloud developing, it is significantly important to obtain sufficient image density avoiding background contamination. The Japanese patent Application Laid Open No. 2004-101933 proposes that when development is executed while forming an advancing wave electric field, an amount of conveyance is controlled by adjusting a frequency of a conveyance voltage or the like. Since the method only adjusts an amount of toner existing in the developing region, and the idea is the same as conventional one or two component development, and accordingly doesn't adjust a clouding condition, it is difficult to control toner uniquely having a weak binding force in the cloud. Thus, a range capable of obtaining sufficient image density without background stein is significantly narrow, or is not obtained depending upon a cloud condition.